This invention relates to color-image data preprocessing for color-image printing, and more specifically, to methodology relating to patterning and dot-gain improvements associated with vector error diffusion processing of color-image data which is to be printed by an electro-photographic (EP) engine. As will be seen, preferred practice of the present invention is performed in the context of a multi-color-level EP engine, referred to generally as multi-bit (N) color output device. Such a device is specifically illustrated herein in the form of a 2-bit EP engine.
In the processing of color-image data, error diffusion practice often gives rise to what is known as high-frequency blue noise, like halftone patterning. This behavior usually contributes to undesirable graininess and other patterning problems, such as so-called “melting”, or “tailing”. Additionally, when a processed result is printed by an EP engine, there is often relatively severe dot-gain which can produce a very undesirable image.
In prior art attempts to deal with these problems, often employed is a thresholding approach which is used to control pixel placement and to avoid undesirable pixel clustering. However, this approach can lead to further undesirable qualities in an image, such as color shifting and other types of unwanted patterning. All of these issues tend to become magnified in a setting where vector error diffusion is performed.
The present invention addresses these issues in a relatively simple, yet significantly effective manner. More specifically, the unique approach to color-image-data pre-processing for printing which is taken by the present invention soidly addresses these issues by using at least two different color palettes to handle color pixel processing. If desired, and as will become apparent to those skilled in the art, more than two different color palettes could be used if desired.
According to a preferred manner of practicing the invention, such practice specifically employs, as illustrated herein, two quite different-character color palettes. One of these palettes is a bi-tonal palette, or look-up table (LUT), and the other is a multi-level (multi-bit), 2-bit palette, or LUT. For illustration purposes, input image-data color space is chosen to be sRGB, and the LUT-converted-to color space is basic CMYK color space for the bi-tonal LUT, and (as will be explained further in detail below) 2-bit C, CI, CII, M, MI, MII, etc. multi-color-level color space for the 2-bit LUT.
Still referring to preferred practice of the invention, a 2×2 4-bit cluster (a half-tone cell) of pixels is chosen to define what are referred to herein, in two different “scan lines”, as high-priority and low-priority pixels-two each in each cluster. Scan line number One which, as will be seen, is the upper one of the two scan lines contains the two, designated, high-priority pixels. Scan line number Two, the lower scan line, contains the two, designated, low-priority pixels.
In the practice of this invention, and in relation to the performance of pre-printing vector error diffusion, the high-priority pixels are processed using the multi-level LUT, and low-priority pixels are processed using the bi-tonal LUT.
This use of such differentiated LUTs is a key contribution of the present invention. Implementation of the invention, as suggested earlier, significantly addresses the undesirable EP printing issues which have been mentioned above, and definitively offers improvements relating to image pixel patterning and dot-gain in finally printed color images.
Various other features and advantages which are offered by the invention will become more fully apparent as the detailed description which follow is read in conjunction with the accompanying drawings.